THEORETICAL BASIS FOR THE SEQUENCE ANALYSIS OF THE REACTION STEPS INVOLVED IN THE TRANSPHOSPHORYLATION PROCESS OF THE SARCOPLASMIC CA++-TRANSPORT ATPASE

Abstract

The splitting of adenosine triphosphate (ATP) by the calcium-transporting ATPase of the sarcoplasmic reticulum (SR) occurs in two steps: (1) the phosphorylation of the enzyme by ATP and (2) the hydrolysis of the phosphoenzyme. Both steps are fully reversible. The actual enzymatic activity of the calcium transporting enzyme is the ATP–ADP-exchange reaction in the steady state. This activity is measured as the incorporation rate of radioactive adenosine diphosphate (ADP) into the ATP fraction under the influence of the enzyme. The assays for the measurement of ATP–ADP exchange contain ATP and ADP as the substrates and magnesium and calcium as divalent cations. The chapter discusses the binding steps of Mg ++ and substrates to the transport enzyme, identification of the Ca O -binding step and the sequence of events when calcium is transported through the membrane of the sarcoplasmic reticulum under (quasi-)physiological conditions.